Does the equatorial ionospheric peak electron density really record the lowest during the recent deep solar minimum

The solar activity in 2008—2009,located at the minimum phase of solar cycle 23/24,was unusually low,which attracts ionospheric physics scientists to explore the plasma behaviors in the ionosphere during the deep solar minimum.Some investigators reported a reduction in ionospheric electron density during this period,while others found a marginal and minor change.In this study we collected the critical frequency of the F2-layer ionosphere(foF2) data retrieved from ionogram records observed by an ionosonde at Jicamarca(12°S,76.9°W),an American station located near the dip equator,to examine the changes in electron density in equatorial region and explore what caused the inconsistence between the differences of solar minimum foF2 in published works.We determined the foF2 differences in moving yearly,seasonal and monthly medians and the Fourier series analysis.The picture of ionospheric changes is found to be related with the data analysis method used.The seasonal median and moving yearly values of foF2 are smaller in 2008—2009 than in 1996—1997,both in the daytime and nighttime.In contrast,the monthly median foF2 is found to be of varying solar minimum-to-minimum differences.Although greater values in the last minimum are prevailing in the monthly median case at most local times,opposite changes are also found at some time intervals.Further analysis reveals that the reduction in foF2 during the current solar cycle minimum is certainly presented in the longer time scale variations of foF2.Therefore,the inconsistent changes in the published investigations reflect the impact of foF2 variability on the solar minimum-to-minimum difference over different time scales.